Structural and chemical techniques are being used to elucidate the mechanism of mineralization in calcified molluscan bivalve hinge ligaments. This tissue is organized into two prismatic forms, pseudo-prisms and noded prisms. The prism contain oriented aragonite crystals (100 nm in diameter), widely and regularly spaced (about 100 nm separation), in an elastic protein matrix. The pseudo-prisms are cylindrical units oriented perpendicular to the ligament growing surface like the apatite prisms of dental enamel. The noded prism are cylindrical but constricted or noded at regular intervals. The aragonite crystals of both prisms are surrounded by a protein sheath which is distinct from the bulk elastic matrix. A homogenous preparation of these sheaths been isolated from the ligaments of Spisula solidissima and Mercenaria mercenaria. Chemical studies on the isolated sheaths will include assays for a) Ca 2 ion binding and carbonic anhydrase activity, and b) the ability of sheaths to bind to crystalline CaCO3 polymorphs and to direct the formation of the aragonite polymorph in the presence of suitable ions. Electron microscopy of carbon replicas of a) the ligament's growing surface and b) the adjacent mantle cells which secret it, will be used to determine and describe the phases in the formation of the ligament. Electron microscopy and electron problem analysis will be used to determine the initial composition of the amorphous mineral deposits in new prisms. Comparative studies on ligaments from a) species that have a calcified ligament and b) species that have an uncalcified ligament will be used to elucidate in part the factors required for mineralization.